Breaking transmissions?

[wannaS13]

Ok so i have a 92 240 coupe with an sr with a s15 t28 and supporting mods. i have not dynoed it yet but i assume it is around 300hp/300trq. just in this summer alone i have broken a set of axles and two transmissions ( it actually snaps 6-8 inches of the tail shaft off). i am not dropping the clutch i am slipping it as best as possible with an act solid race xtreme 6 pad clutch. i am so sick of this thing breaking dont know what to do i have only been able to get a 12.8 out of it and i know ot has so much more in it. i had a weled diff but took it out after the axles as i thought that was the problem. any ideas as to why its breaking or what i should do next?